The present invention is generally related to fuel control systems, and, more particularly, the present invention is related to electronic fuel injection (EFI) control systems and techniques for adjusting injection timing control of compression ignition engines to reduce exhaust emissions and/or improve engine efficiency.
Relatively large compression ignition engines, such as diesel engines used in locomotive or marine applications, may recently have the ability to vary start of injection timing (timing). See U.S. Pat. No. 5,394,851 for a description of an EFI system that provides variable injection timing to such engines. For example, as represented in FIG. 1, the injection timing may be obtained from a two-dimensional look-up table that in concept should map a respective timing value as a function of engine speed and a fuel value or quantity to be delivered to each cylinder of the engine. Unfortunately, since the mapping is executed in finite increments of engine speed and fuel values, most operating points need a double interpolation to calculate timing for a given set of engine speed and fuel values.
As will be appreciated by those skilled in the art, a locomotive throttle may be operable at a plurality of different power positions or notches. For example, there may be eight power positions or notches (N), plus idle. N1 may correspond to the minimum desired engine speed or power, while N8 may correspond to maximum speed and full power. In general, a notch specifies a commanded engine speed and power and nominally would define a point in the two-dimensional look-up table to get a suitable injection timing. Although the concept of mapping is straight forward, there are some practical considerations that have generally prevented obtaining a unique injection timing value for a given notch. First, there may be more than one notch for a given engine speed and this causes such notches to share the same area in the map. Second, the fuel value may depart or deviate from a nominal timing value, e.g., nominal operating point 7, due to various external variables, such as ambient temperature, atmospheric pressure, fuel temperature, fuel quality, fuel injection equipment wear, variation from locomotive-to-locomotive, etc. The foregoing fuel value deviations transform what originally should have been a single operating point into a band of possible operating injection timing values, e.g., operating band 8. The band size may be relatively large since the fuel value deviations may be correspondingly large. The above-described conditions result in operating points that overlap, as represented by the dashed ovals in FIG. 1, and do not uniquely identify a respective timing value for each respective notch.
With the introduction of relative stringent exhaust emission regulations on locomotives, it becomes particularly desirable to be able to set a unique fuel injection timing value at each notch. Although reducing the size of the increments between the engine speed and fuel values would somewhat reduce the amount of overlap in most conditions, this approach would require a two-dimensional table that would require significantly more memory and processing power in a controller. Another approach would be to design the EFI system so that the engine runs the same timing in all notches that are known to overlap. Unfortunately, such approach does not provide an optimum solution for engine performance and reduction of exhaust emissions. Thus, it would be desirable to provide an EFI control system and techniques that, upon recognizing that the engine is in a normal operating condition, as opposed to a transient operating condition or strenuous environmental conditions, allow for assigning a unique injection timing value to each respective notch. It is also desirable to be able to adjust the set timing based on the values of predetermined variables capable of affecting the characteristics of the combustion process so as to increase engine efficiency and reduce respective levels of exhaust emissions, such as nitrogen oxides (NO.sub.x) and other pollutants. Examples of such variables may include atmospheric or barometric pressure, manifold air temperature, oxygen concentration, humidity levels, fuel type, engine type, equipment age, etc.